A process for producing unsaturated aldehydes and/or unsaturated acids from olefins is a typical example of catalytic vapor phase oxidation.
In general, catalytic vapor phase oxidation is implemented as follows. At least one catalyst in the form of granules is packed into reaction tubes, feed gas 1 is supplied to a reactor through the reaction tubes and the feed gas is in contact with the catalyst in the reaction tubes to perform vapor phase oxidation. Reaction heat generated during the reaction is removed by heat transfer with a heat transfer medium, wherein the temperature of the heat transfer medium is maintained at a predetermined temperature. Particularly, the heat transfer medium for heat exchange is provided on the outer surface of the catalytic tubes to perform heat transfer. A reaction mixture 3 containing a desired product is collected via a duct and then sent to a purification step. Generally, catalytic vapor phase oxidation is a highly exothermic reaction. Therefore, it is very important to control the reaction temperature in a specific range and to downsize hot spots in the reaction zone.
To perform the partial oxidation of olefins, a multimetal oxide containing molybdenum and bismuth or vanadium or a mixture thereof is used as a catalyst. Typically, the partial oxidation of olefins may be exemplified by a process for producing (meth)acrolein or (meth)acrylic acid by oxidizing propylene or isobutylene, a process for producing phthalic anhydride by oxidizing naphthalene or ortho-xylene or a process for producing maleic anhydride by partially oxidizing benzene, butylene or butadiene.
Generally, propylene or isobutylene is subjected to two-step catalytic vapor phase partial oxidation to form (meth)acrylic acid as a final product. More particularly, in the first step 10, propylene or isobutylene is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrolein 2 as a main product. In the second step 20, (meth)acrolein obtained from the preceding step is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrylic acid 3. The catalyst used in the first step is an oxidation catalyst based on Mo—Bi, which oxidizes propylene or isobutylene to form (meth)acrolein as a main product. Additionally, a part of (meth)acrolein is further oxidized on the same catalyst to form acrylic acid partially. The catalyst used in the second step is an oxidation catalyst based on Mo—V, which oxidizes (meth)acrolein-containing mixed gas produced in the first step, particularly (meth)acrolein, to form (meth)acrylic acid as a main product.
Reactors for carrying out the above process are realized in such a manner that each of the above two steps are implemented in one system or in two different systems (FIG. 1) (see U.S. Pat. No. 4,256,783).
Meanwhile, many attempts are made to increase productivity of the reactor for producing acrylic acid by modifying the reactor structure, suggesting an optimized catalyst for oxidation or improving the operational conditions.
As mentioned above, vapor phase oxidation of propylene, isobutylene or (meth)acrolein is an exothermic reaction. Therefore, it has a problem in that a hot spot (a point whose temperature is abnormally high or where heat accumulation is relative high) is generated in a catalytic bed in the reactor. Such hot spots show a relatively high temperature compared to other parts of the reactor. Accordingly, in hot spots, complete oxidation proceeds rather than partial oxidation, thereby increasing by-products such as COx, and decreasing the yield of (meth)acrolein or (meth)acrylic acid. Further, the exposure of catalyst to high temperature causes rapid inactivation of catalyst, thereby shortening the lifetime of catalyst. To solve these problems, a method for inhibiting the generation of hot spots and equalizing the availability of catalyst over the whole reactor has been studied to obtain (meth)acrolein or (meth)acrylic acid with high yield and to use the catalyst for a long time. In this regard, many improved catalysts have been continuously suggested.
For example, Japanese Laid-Open Patent Nos. Sho43-24403 and Sho53-30688 disclose a method for packing a catalytic bed by diluting a catalyst with an inactive material in a stepwise manner from the inlet of feed gas to the outlet of feed gas. However, the above method has a problem in that it takes too much time and is very difficult to pack the catalytic bed while varying the dilution ratio with an inactive material from 100% to 0% gradually. In addition, Korean Laid-Open Patent No. 1997-0065500 and Japanese Laid-Open Patent No. Hei9-241209 disclose a method for packing a catalytic bed by controlling the volume of finally formed catalyst (secondary particles) in such a manner that the volume gradually decreases from the inlet to the outlet. However, the above method has problems in that when the finally formed catalyst has a relatively large volume, reaction tubes may be obstructed, and that it is not possible to obtain a desired level of conversion into acrolein and yield of acrylic acid, due to insufficient activity of such large catalyst. Further, Korean Laid-Open Patent No. 2000-77433 and Japanese Laid-Open Patent No. 2000-336060 disclose a method for using multiple kinds of catalysts formed by varying the kind and amount of alkali metals. However, the method has a difficulty in producing catalysts having different activities at a correct ratio because the amount of alkali metal used therein is small. Further, Japanese Laid-Open Patent No. 2003-171340 discloses a method for using multiple kinds of catalysts formed by varying particle diameters of silicon/carbon compounds (carrier). When the catalytic activity is controlled by varying the particles of SiC, particle size of the SiC (used as a carrier) can decreases and however, it is difficult to produce catalysts having different activities by a desired degree, because such decreased carrier particle size bears no relation to the primary particle size of a catalytically active component.
Therefore, there is a continuous need for developing a method for producing unsaturated aldehydes and/or unsaturated fatty acids with high yield and using a catalyst stably, by controlling the temperature of the highest hot spot efficiently.